small ECU in Honda

Development of the ECU-integrated throttle body module

To allow for application to a small motorcycle, the FI system had to be simplified and down-sized considerably from the conventional one. The development team packed various functions into the compact yet simple system that could be installed in the place of a conventional carburetor by modularizing the throttle body, various sensors, and the engine control unit (ECU) into an integral unit.

Aims of integrating and modularizing the throttle body and ECU

In conventional motorcycle FI systems, the ECU is mounted to the vehicle body. Also, various sensors for detection of the control information are located at various places in the engine or vehicle body as an independent component. Because of that, many wire harnesses are required to connect various sensors to the ECU. Consequently, the conventional system is complex, and difficult to apply to a small motorcycle having a 50cc-125cc single cylinder engine. In an attempt to practically apply a compact PGM-FI in place of a carburetor, the throttle body, which controls the intake air volume, and the ECU module, which controls engine operating conditions, have been integrated into one unit. With the wire harnesses eliminated, the size of the module has been reduced to the same size as a carburetor.

Outlines of system

The newly developed compact PGM-FI system has a two-split configuration consisting of the throttle body and the ECU module. This configuration allows for replacement of a throttle body having a bore size suitable for a particular engine displacement, thus realizing a FI system applicable to various models with a high level of application freedom. The throttle body and ECU module incorporating various sensors are connected with 4 bolts. The idle air passage is grooved on the mating surface, and by sealing with an O-ring, the idle air passage is formed, thus contributing to the integration and down-sizing. The ECU module unit consists of the plastic box (device body), ECU board, and the cover. Various sensors are housed in the device body, and the ECU board is connected directly to the input and output terminals of the sensors. Applying the cover from above, the inside is packed with potting resin to secure the internal parts and to prevent water entry.

Layout of sensors

The throttle body module has 3 sensors in the device body.

Intake air temperature (Ta) sensor: To allow measurement of intake air temperature, the sensing tip of the sensor is exposed in the intake air passage before the throttle valve. For down-sizing, the sensor terminals are directly mounted on the ECU board. ECU module component

Throttle position sensor (TPS): Located on the end of the throttle shaft, directly detects the throttle opening. The throttle shaft and the sensor rotor are connected via a spring to eliminate a hysteresis in the operation. The TPS is directly fit into the device body and sealed from outside by potting.

Manifold vacuum (Pb) sensor: The sensor terminals are directly mounted on the ECU board, and connected to the connecting passage provided after the throttle valve in the throttle body.

ECU board

The ECU board used in the PGM-FI system for small motorcycles is exclusively designed for single cylinder engines. The size is reduced by providing an injector driver circuit and an ignition circuit for one cylinder, allowing mounting on the side of the throttle body. The CPU used for the controller is a 16bit CPU. The large parts such as the power supply condenser are located in the space between various sensors in the device body, contributing to reduce the overall width to the same level as a conventional carburetor. The ECU board is a 4-layer structure to reduce the surface area for various circuits. In addition, the harness connector, which takes a large amount of space in a conventional ECU, has been down-sized to approximately 1/2 from the conventional one by using a terminal-to-terminal pitch of 2.6mm·32 pin design. The reduction of terminal-to-terminal pitch becomes possible by using an adhesive gel sheet to seal the connector from water. To allow application of additional function such as an immobilizer, the number of pins is set at 32.

Idle air control device

Compared to a large motorcycle, less volume of intake air has to be controlled for a small displacement motorcycle engine. To cope with the stringent exhaust emission regulations, delicate control capabilities are also required. At the same time, to make an FI system applicable to small motorcycles, down-sizing is the key. To satisfy these demands, the slide-valve-type air control valve (SACV) driven by a stepping motor is applied to the PGM-FI system for small motorcycles. In conventional direct drive type, a stepping motor of ø20mm was required to maintain the operating torque to overcome the intake vacuum. In this system, the size of the stepping motor has been reduced to ø14mm by using the slide valve design.

FI system control

The PGM-FI system for small motorcycles controls the fuel injection volume, injection timing and the ignition timing based on signals from the throttle position sensor (TPS) in the ECU module, the manifold vacuum (Pb) sensor, and the crank position sensor that detects the rotation angle. The fuel injection volume, the injection timing and the ignition timing are further compensated by the engine temperature, intake air temperature and the atmospheric pressure to ensure optimum controls under various environmental conditions.

Fuel injection control

For the control of fuel injection volume, two kinds of maps are stored in the ECU and an appropriate map is selected and used depending on the throttle opening and the engine revolutions. 1. When the loads are low, the delicate changes in throttle opening are detected by the manifold vacuum, and the manifold vacuum map determined by the manifold vacuum and the engine revolutions is used. 2. When the loads are high, the throttle map determined by the throttle opening and the engine revolutions is used.

Ignition timing control

For ignition timing, the map control determined by the throttle opening and the engine revolutions is executed to control the ignition timing at the optimum timing.

O2 feedback control

To efficiently use the 3-way catalyst in the exhaust muffler, the feedback system using an O2 sensor is applied to control the mixture near the stoichiometric ratio.

Control of idle air control device

The idle air control valve regulates the intake air volume depending on the operating conditions such as the starting, warming up and idling. When starting, the intake air is supplemented by opening the idle air control valve depending on the engine temperature for easy starting. The opening of the air control valve is regulated depending on the increase of the engine temperatures to control the intake air volume at the optimum level. After the engine reaches the prescribed temperature, the intake air volume is controlled to maintain idling at a constant speed by the revolution-feedback control. This feedback control eliminates the conventionally required idle speed adjustment, thus eliminating the need of maintenance to compensate for the secular distortion.